In the optical communications, in order to achieve efficient utilization of the optical fibers, a WDM transmission system, which achieves high-speed optical transmission with a single optical fiber by multiplexing a plurality of optical signals of different wavelengths, has come to be employed more and more. Further, the use of a dense WDM (DWDM) which enables still higher-speed transmission by multiplexing optical signals of several tens of different wavelengths has been spread.
Further, ROADM (Reconfigurable Optical Add/Drop Multiplexer) which adds/drops optical signals of an arbitrary wavelength at each node has been studied for practical use. When this ROADM system is employed, it becomes possible not only to expand the transmission capacitance by wavelength multiplexing but also to switch the optical paths by changing the wavelengths. Thus, versatility in optical networks is expanded drastically. In this case, light sources corresponding to each wavelength are required in an optical communication network system, so that the number of required light sources is increased drastically in accordance with high multiplexing.
A wavelength variable laser device shown in FIG. 6 is known as a light source for the WDM transmission system (Patent Document 1, for example). Hereinafter, explanations will be provided by referring to this drawing.
A wavelength variable laser device 70 includes: a double ring resonator 72 formed on a PLC 71; an SOA 73 which supplies light to the double ring resonator 72; a high-reflective coating 74 that returns the light transmitted through the double ring resonator 72 to the SOA 73 via the double ring resonator 72; and optical waveguides 75 and 76 which are formed on the PLC 71 and connect the SOA 73, the double ring resonator 72, and the high-reflective mirror 74. A double ring resonator 82 is formed with: ring resonators 77 and 78 having different optical path lengths from each other; and an optical waveguide 79 which connects the ring resonators 77 and 78. Film-like heaters 80a, 80b, 81a, and 81b for changing the wavelength of the light transmitting through the ring resonators 77 and 78 are provided on the ring resonators 77 and 78.
As described above, the wavelength variable laser device 70 is in a structure in which the double ring resonator 72 is structured on the PLC 71, and the SOA 73 is directly mounted onto the PLC 71. The circumferences of the two ring resonators 77 and 78 formed on the PLC 71 are slightly different from each other. The Vernier effect occurs due to the difference in the circumferences, so that it is possible to obtain output light 82 with a wide variable range of wavelengths.
It is necessary with typical wavelength variable lasers to stabilize oscillation by optimizing the phase condition of laser resonators in regards to the oscillated wavelengths, so that a phase adjustment mechanism is employed. For example, with a monolithic-type wavelength variable laser, a semiconductor layer of a composition wavelength having no optical gain for the oscillation wavelength is provided as a phase control region within the optical waveguide, and the refractive index of the semiconductor layer is changed by implanting electric currents to the phase control region so as to control the phase condition at the time of laser resonance.
In the meantime, the wavelength variable laser device 70 is provided with the SOA 73 having a phase control region 83 to change the refractive index of the optical waveguide of the phase control region 83 electrically to achieve a function that is equivalent to the phase adjustment mechanism of a monolithic type.
FIG. 7 is a schematic sectional view showing the SOA of the wavelength variable laser device shown in FIG. 6. Hereinafter, explanations will be provided by referring to FIG. 6 and FIG. 7.
The SOA 73 includes: a laminate body formed with a p-type semiconductor layer 84, active layers 85, 86, and an n-type semiconductor layer 87; electrodes 88, 89, and 90 provided on a voltage applying surface of the laminate body; and a low-reflective coating 91 and a non-reflective coating 92 provided on a light-emitting surface of the laminate body. Further, the SOA 73 is separated into a gain control region 93 and the phase control region 83 in terms of the functions thereof. The gain control region 93 has an exclusive electrode 88 and an exclusive active layer 85. The phase control region 83 has an exclusive electrode 89 and an exclusive active layer 86. The active layer 86 is of a different composition from that of the active layer 85, and emits a composition wavelength having no optical gain for the oscillation wavelength.
In addition to the phase control function of the double ring resonator 72, the phase control region 83 also has a phase modulation function for suppressing stimulated Brillouin scattering (referred to as “SBS” hereinafter) generated in the optical fiber as the output target. In the phase control region 83, a plasma effect is generated by implanting carriers to the active layer 86, and changes in the refractive index caused thereby are utilized to achieve the phase control function and the phase modulation function.    Patent Document 1: Japanese Unexamined Patent Publication 2006-245346